@innogen - it doesn't come with calibre, you have to install it using Preferences->Plugins->Get New Plugins. And it is specifically called "Modify ePub". When you install it you will be asked which toolbar(s) you want to add it to, and after then restarting calibre you will have a shiny new icon on that toolbar you selected to use it. See the Plugins subforum (specifically the sticky posts at the top) for more info about plugins.